Two phosphorylated and two non-phosphorylated variants of P protein isolated from Newcastle disease virions are known. Here, a fifth form of P was identified using two-dimensional polyacrylamide gel electrophoresis and peptide mapping. P form 5 was phosphorylated; however, unlike the four known variants of P, the new form was not a major protein in virions, which suggested an intracellular function. The subunit composition of four electrophoretically distinct, disulphide-linked multimers of P from virions was determined. Each homomultimer was composed of at least three molecules of a different one of the four virion-associated P variants.